phototropism (n.)

word-forming element meaning "light" or "photographic" or "photoelectric," from Greek photo-, combining form of phōs (genitive phōtos) "light" (from PIE root *bha- (1) "to shine").

1899, "tendency of an animal or plant to turn or move in response to a stimulus," 1899, abstracted from geotropism or heliotropism, with the second element taken in an absolute sense; ultimately from Greek tropos "a turning" (from PIE root *trep- "to turn").